This invention relates to improved photodetector arrangements and circuits, and more particularly, photodetector arrangements and circuits in which the light beam strikes or passes through one or more photodetectors more than once.
In a number of analytical instruments, such as spectrophotometers, a photodetector is employed to convert photon energy from a light beam into electrical energy by photoelectric effect and the resulting electrical energy is measured to determine certain characteristics of the light beam and/or characteristics of the material from which the light beam has emanated before reaching the instrument.
The present invention is in improved arrangements of such photodetector elements and their circuits to substantially improve the efficiency of the photodetectors and to increase the spectral response range of the analytical instrument in which they may be used. In photodetector arrangements and circuits incorporating the principles of the present invention, phototubes may be used which might otherwise have previously been rejected for low energy response. Another advantage of the arrangements and circuits incorporating the principles of the present invention, is that the wavelength range of prior instruments may be extended to as much as 1000 nm without the addition of moving parts, without drastic modifications to preexisting equipment and without substantially increased expense. By extending the wavelength range of such prior spectrophotometers, it is possible to analyze for phosphates and other chemical compounds which are outside of the normal maximum 800 nm wavelength range of prior spectrophotometers. Another advantage of arrangements and circuits incorporating the principles of the present invention is the realization of increased signal-to-noise ratios of, for example, in excess of 2,000:1 at 2A. Such improved signal-to-noise ratios maximize the quantum efficiency of the equipment and render the analytical instruments useful in monitoring small changes at high absorbence or trace components at low absorbence which were previously difficult or impossible to analyze.
To one embodiment of the present invention, an improved photodetector arrangement comprises photodetector means having a cathode which is at least semitransparent to a light beam and is capable of emitting electrons when positioned in the light beam, wherein the cathode is positioned relative to the light beam so that the light beam passes through the cathode at least twice.
In another embodiment of the present invention, return means is positioned relative to the photodetector means for returning the light beam to the cathode after the light beam has passed through the cathode twice so that the light beam strikes the cathode at least a third time.
In still another embodiment of the present invention, two photodetector means are positioned relative to each other so that the light beam, after striking the first photodetector, strikes the second photodetector.
And in still another embodiment of the present invention, at least two photodetectors are electrically coupled in parallel to each other.
These and other objects, features and advantages of the present invention will be more clearly understood through a consideration of the following detailed description.